This invention relates to a heater which may be used as a primary heat source or as an auxiliary heat source for use with a further heat source. In either use, the invention is suited, inter alia, for the installation and/or removal of modular electronic components with respect to a printed circuit board (PCB) or the like. Moreover, the invention relates to circuitry for controlling the heater and heat sensing elements that may be associated therewith.
Present day devices for removing or installing modular surface mounted devices (SMD's) from a substrate such as a printed circuit board include those which use heated, flowing air to melt the solder. In particular, these devices direct the flow of the heated air at the component terminals from a primary heated air source disposed above the component to simultaneously melt the solder at the terminals. Such a device, for example, is disclosed in U.S. Pat. No. 4,366,925 and co-pending application Ser. No. 649,065, filed Sept. 10, 1984, both the foregoing patent and application being incorporated herein by reference. Since in some applications, the components or substrates may be subjected to thermal shock due to the temperature difference between the solder melt temperature of the heated air and the ambient room temperature of the component, it is desirable to provide a preheater positioned beneath the printed circuit board and, in particular, the *component to be removed or installed, to preheat the board to a temperature below the solder melt temperature but well above room temperature. Such a preheater is generally disclosed in the above-mentioned co-pending application.
Other considerations relating to the desirability of preheating the printed circuit board and component are (a) many components and printed circuit boards are heat sinked and thus, in order to facilitate the task of the primary heater, a preheater can raise the temperature of the heat sinked component or board to a temperature substantially higher than the ambient temperature to thus avoid the necessity of overdriving the primary heater to overcome the heat draining property of the sink and (b) it is desirable the solder melt heat applied from the primary source be applied as quickly as possible for several reasons including lessening the danger of damaging adjacent components. Thus, by initially raising the temperature to the preheat temperature, the time the solder melt heat is applied can be lessened.
As will be described further below, not only is a preheater of the above type provided in accordance with the present invention, but also a heater which may be used as a primary heat source in place of the hot air source described above.
In addition to devices for installing and removing SMD's, there are those which are directed to the installation and removal of through-hole mounted devices--that is, the leads of which extend through holes in the PCB. Many devices of this type employ a molten solder bath. One such device is described in co-pending application Ser. No. 763,704 filed Aug. 8, 1985, this application being assigned to the assignee of the subject application and is hereby incorporated by reference herein. In accordance with another aspect of the invention, through-hole mounted deivces (and, in certain applications, even SMD's) may be installed or removed from a printed circuit board or the like using a heater in accordance with the present invention which does not utilize molten solder or the like.
When employed as a preheater for a printed circuit board, the heating element should be brought into intimate contact with the underside of the board to insure good heat flow from the element to the board. However, the underside of the board typically includes at least terminals, pads, and connections and may also include circuit components assuming components are mounted on both sides of the board. Thus, the underside of the board will be uneven and intimate contact may be difficult to achieve. As will be described below, various measures are provided in the present invention to accommodate unevenness on the board underside.
In a heater of the type employed in the present invention, an electrical current is passed through a heating element, such heaters, in general, being well known. Circuits are also known which control the temperature of the element in response to a heat sensing element, which measures for temperature of the heating element or a heat transfer member in intimate contact with the heating element. In general, such systems compare the desired temperature of the heating element (which is usually externally set by an operator) with the temperature measured by the heat sensing element. The current through the heating element is then turned on and off in response to the measured temperature being greater or less than the desired temperature, respectively. In this manner, an attempt is made to maintain the heating element at an equilibrium temperature corresponding to the desired temperature as the heating element loses heat to its surroundings and gains heat from the electrical current source.
Such heater control circuits are subject to several shortcomings. In devices of the above type, the desired temperature is usually set by adjusting a potentiometer, the magnitude of the voltage output of which corresponds to the desired temperature. Moreover, the temperature of the heating element is memasured by a transducer which converts the measured temperature to another voltage, the magnitude of which corresponds to the measured temperature. Due to inaccuracies in the circuits for providing the above voltages, the voltage output of the transducer may not in actuality correspond to the desired temperature of the heating element when an indication is given by the circuitry that the desired temperture has been reached. This difference between the actual temperature and the desired temperature of the heating element can, in many instances, be substantial and thus the heat delivered by the heating element to a work can deviate substantially from what is required to do the job. In such instances, a display of the measured voltage may be 280.degree. F. while the potentiometer may be set to 250.degree. F. Needless to say, the operator is at a loss as to which temperature value is correct.
In accordance with one feature of the invention, improved circuitry is provided which insures the temperature set by the operator will be substantially the same as that measured by the heat sensing element. Moreover, improved circuitry is provided for facilitating calibration of the circuits which set the desired temperature and which measure the actual temperature where the number of components and the amount of time needed to effect the multiple calibrations are minimized.
When the operator sets a desired temperature this is typically a desired temperature at some point or area on the work and not on the heating element itself. In many applications, the temperature differential between the heat element and the point or area on the work can be very substantial. Thus, this can be a further reason that the heat delivered to the work is not that required for the job.
Improved circuitry is also provided, in accordance with a further aspect of the present invention, where the heat delivered by the heating element is such that, when an operator sets a desired temperature for a point or area on the work, the actual temperature at that point or area will correspond to the desired temperature set by the operator.
Although the above discussion has been in terms of measuring temperature, other physical parameters such as pressure may also be advantageously measured or controlled with the circuitry of the present invention.
When measuring temperature, the foregoing procedures may include determining the thermal characteristics such as thermal conductivity, heat capacity, etc. of the work. Assuming the work is a standard PCB, for example, these thermal characteristics, once determined with circuitry of the type employed in the present invention, will be known for all boards of the above type. In accordance with a further aspect of the invention, these known thermal characteristics may then be utilized to automatically determine, under microprocessor control, for example, the amount of heat needed to raise a point or area of the work to a desired temperature. Moreover, the process, by which the heat is delivered to the work may vary--that is, it may comprise electrical current heating, molten solder, heated air, etc. Again under microprocessor control, for example, the parameters characterizing each of these processes may be varied to ensure the delivery of the proper amount of heat to a desired point or area of the work in response to the thermal characteristics of the work, the characteristics having been predetermined as described above.
Other objects of the invention include the provision of an improved heater, which may be used as a preheater of the type aforementioned, although it has many other uses other than preheating a printed circuit board, as will become apparent from the following description.
Another object of this invention, when the heater of this invention is used as a preheater for preheating a printed circuit board or the like, is that it includes a platen which is mounted so as to wobble and therefore come into alignment with the bottom surface of a printed circuit board when the platen is placed against the underside of the printed circuit board. The platen also is self-leveling and therefore conforms to the level of the printed circuit board against which it is placed.
Another object of the invention is to mount the platen on a shaft which, in turn, is mounted in a bearing with sufficient play between the shaft and the bearing that the platen may wobble, as described above. The shaft further permits locking the platen in a fixed position thereby preventing any further wobbling of the platen for use as a primary heating surface.
The mounting of the platen such that it may wobble permits the platen to contact the printed circuit board on the underside thereof at all points, whereby even heating may be applied to the board and all portions thereof, even though the printed circuit board may not be level. This feature insures that a component to be removed from the board will be evenly heated from the underside thereof, which facilitates a clean removal of a component from the board without damage to the leads of the component.
Another object of the invention is to provide various compliant means such as heat conductive layers or beads to effect heat transfer from a heated platen to an uneven surface of a work such as the underside of a PCB.
Another feature of the heater of this invention when used as a preheater is its use with printed circuit boards having components on the both sides thereof. Assuming a component to be removed or installed is disposed on the upper side of the board and the preheater is disposed at the underside of the board, compliant projections extending upwardly from the platen or a plate disposed on the platen may be employed which engage the underside of the board at locations where no components are disposed. In this manner a board of the above type may be selectively preheated to facilitate removal or installation of the component on the upper side of the board.
In accordance with a further aspect of the invention the platen or heating element may be divided into a plurality of zones which deliver different amounts of heat to the work, the work and platen being relatively movable with respect to one another whereby a first zone of the platen may preheat the work and a second zone may deliver primary heat such as that necessary to effect either solder melt or joint formation.
In accordance with a further feature of the invention, a temperature probe or the like may be employed to measure the temperature at the upper surface of the board or at another suitable location to provide an indication of the upper surface temperature. This measurement may also be employed in a control loop including the heater to automatically maintain the board temperature at a desired preheat temperature or other desired temperature.